It is well known that a direct positive photographic image can be formed on a silver halide photographic material without employing any intermediate processing step or forming a negative photographic image. Aside from any special techniques, the conventional methods that are practically useful for the purpose of forming a positive image on direct-positive type silver halide photographic materials are divided into the following two major types: in one type, an emulsion containing a prefogged silver halide is used and development for the formation of a positive image is carried out by destroying fog (latent-image) centers in the exposed areas by making use of solarization or the Herschel effect; and in the other type, an amulsion containing a non-prefogged internal image forming silver halide is used, and after image exposure, a positive image is produced by performing surface development after and/or during fogging.
In the second type of direct positive image forming method, fogging of the exposed silver halide is performed either by applying overall exposure within a developer or a prebath, or by using a foggant. The term "emulsion containing an internal image forming silver halide" means a silver halide emulsion that has sensitivity specks predominantly in the interior of a silver halide grain and which forms a latent image in the inside of the grain upon exposure. The second type of direct positive image forming method generally attains a higher sensitivity than the first type and, hence, is suitable for use in applications requiring high sensitivity.
Methods of forming a direct positive image on a photographic material using an emulsion containing an internal image forming silver halide by performing, after imagewise exposure, surface development either by applying overall exposure or by using a foggant, as well as the photographic emulsions or photographic emulsions used in such methods are described in U.S. Pat. Nos. 2,456,953, 2,497,875, 2,497,876, 2,497,917, 2,507,154, 2,588,982, 2,563,785, 2,675,318, 3,227,552, 3,447,927 and 3,511, 662, and British Pat. No. 1,151,363.
No clear-cut explanation has been made of the detailed mechanism underlying the formation of a direct positive image with the use of an emulsion containing an internal image forming silver halide, but a certain picture of the process of the positive image formation can be obtained from the "desensitizing effects of an internal latent image" discussed in, for example, "The Theory of the Photographic Process", fourth edition, ed. by T. H. James, p. 190, Macmillan Publishing Co., Inc. When an "internal latent image" is formed in the inside of a silver halide grain by the first imagewise exposure, said latent image provides the surface desensitizing effect that allows fog centers to form selectively on the surfaces of unexposed silver halide grains, and the surface fog centers are subsequently developed by ordinary surface development, thereby forming a photographic image in the unexposed areas.
Selective formation of fog centers is customarily done either by photofogging involving the applying exposure to the entire surface of the light-sensitive layer or by chemical fogging involving the use of a chemical such as a foggant.
The term "foggant" used hereinafter is intended to mean a fogging agent that enables selective development of internal image forming silver halide grains that have sites for the formation of an internal latent image but which are yet to receive imagewise exposure, rather than the development of silver halide grains having an internal latent image formed by imagewise exposure.
Methods for obtaining a direct positive image by performing surface development of an internal image forming silver halide emulsion in the presence of a foggant, as well as the photographic emulsions or photographic materials used in such methods are described in U.S. Pat. Nos. 2,456,953, 2,497,875, 2,497,876, 2,588,982, 2,592,250, 2,675,318, 3,227,552, and 3,317,322; British Pat. Nos. 1,011,062, 1,151,363, 1,269,640, and 2,011,391; Japanese Patent Publication Nos. 29405/1968 and 38164/1974; and Unexamined Published Japanese Patent Application Nos. 16623/1978, 137133/1978, 37732/1979, 40629/1979, 74536/1979, 74729/1979, 52055/1980 and 90940/1980.
In order to form a direct positive image by the chemical fogging method, contact with the processing solution is essential and the desired effect of a foggant is attained only under extreme pH conditions (.ltoreq.12). This is responsible for an increased chance of deterioration of the foggant by aerial oxidation and the resulting degradation of the fogging effect is substantial. Further problems result from the fact that treatment with the high pH processing solution is time-consuming and painstaking and that the foggant's performance is sensitive to the effects of variations in processing conditions such as the temperature and agitation of the processing solution.
In most of the known techniques described above, improved photographic characteristics are intended to be obtained by improving the technology of preparing silver halide emulsions and few proposals have been made for producing improved photographic characteristics by means of improving the processing scheme.
U.S. Pat. Nos. 4,124,387 and 4,186,009 describe photothermographic materials designed for producing positive images by the dry process but neither patent suggests a method for producing a direct positive image by employing in the heat-developable photographic material an emulsion containing an internal image forming silver halide.
The present inventors made various studies on the formation of a positive image on a silver halide photographic material and found that, if after imagewise exposure of a silver halide photographic material having an emulsion containing an internal image forming silver halide, the surface sensitivity of the silver halide is increased in the substantial absence of water, fog centers are formed selectively on the surface of the silver halide, whereby a positive image is formed. The present invention has been accomplished on the basis of this finding.